The present invention relates generally to knee joint prostheses and, more particularly, to a knee joint prosthesis which provides for partial stabilization of the joint during leg flexion, while minimally constraining "natural" movements of the joint, and while requiring no additional resection of the femur as compared to non-stabilized knee joint prostheses of similar design.
Knee joint prostheses, which have been generally available for a number of years, can be classified into two types. The first type has been referred to as "stabilized" prostheses in which hinge or ball and socket type joints are used as substitutes for the anotomical knee joint In this type of joint, movement of the knee is controlled and constrained by the hinge pin or ball and socket, and little reliance is placed on the surrounding soft tissues (i.e., tendons and ligaments) to stabilize the joint. These joints allow little, if any, anterior-posterior translation, lateral angulation, or rotation, as does the anotomical knee joint. For this reason, such joints are considered to be undesirable, and may be prone to premature failure.
The other type of knee joint prosthesis is generally referred to as a "condylar surface" prosthesis. In this type of joint, the respective bearing surfaces on the femur and tibia are replaced by similarly shaped and positioned prosthetic bearing surfaces, which are separate from and not directly connected to each other. This type of joint relies upon the surrounding tendons and ligaments to hold the joint together and to impart stability to the joint during movements. These joints are relatively light in weight, afford substantial rotational and translational freedom of movement, and require relatively minor resection of the natural bone and disturbances of the surrounding soft tissues during implantation. Examples of these types of prosthetic joints are disclosed in U.S. Pat. Nos. 3,728,742 and 3,774,244.
In situations where the tendons and ligaments surrounding the knee are damaged or are otherwise unable to impart the desired degree of stability to the condylar surface type prostheses, attempts have been made to introduce features which impart some degree of stability to the joint (by virtue of interactions between the joint components), while minimally interfering with the "natural" rotational and translational freedom of movement of the joint. Examples of devices which include features directed to this end are shown in U.S. Pat. Nos. 4,209,861; 4,213,209 and 4,298,992. Each of the devices described in these patents include a tibial component which has a spaced pair of concave bearing surfaces, and a femoral component which has a spaced pair of convex condylar bearing portions which are matingly received and supported by the concave bearing surfaces of the tibial component. The tibial component further has an upwardly extending post located between the bearing surfaces which extends into a box-like recess formed between the condylar bearing portions of the femoral component. In each case, the box-like recess has lateral side walls, anterior and posterior walls, and a superior wall which interact (i.e., contact) the post of the tibial component under certain conditions to restrain relative movements of the tibial and femoral components within desired limits. While such designs have been successful in imparting a degree of stability to this type of joint, the existence of the box-like recess in the femoral component requires resection of a greater amount of natural bone from the femur, and requires a more complicated preparation procedure for the femur prior to implantation. Furthermore, interactions between the tibial post and the surfaces of the femoral box, which take place at a point which is relatively high above the tibial plateau, increases the tipping moment applied to the tibial component of the joint, and can lead to early loosening and failure of the joint. Accordingly, there exists a need for an improved, partially stabilized knee joint prosthesis which retains the freedom of movement characteristic of the anotomical knee joint, which imparts some degree of stability against anterior dislocations of the femur during leg flexion, which is relatively simply to implant, and which requires minimal removal of the natural bone of the femur.
It is an object of this invention to provide an improved knee joint prosthesis which offers the above-noted advantages.
These and other objects are attained in a knee joint prosthesis which comprises a femoral component and a tibial component which are adapted for implantation on the condylar end of the femur and the tibial plateau, respectively. The femoral component has a pair of spaced-apart condylar bearing portions, an anterior intercondylar portion, a posterior intercondylar portion, and an intercondylar opening defined by lateral, posterior, and anterior edges of the condylar bearing portions, the anterior intercondylar portion, and the posterior intercondylar portion, respectively. The edges which define the intercondylar opening are substantially coplanar such that a box-like recess or structure extending into the intercondylar portion of the femur is not formed (or required) by the joint of the present invention. The tibial component has a pair of spaced-apart bearing surfaces for supporting the condylar bearing portions of the femoral component, and a relatively low intercondylar eminence disposed between the bearing surfaces. The intercondylar eminence has a posterior surface and a peak which defines the highest portion of the tibial component, as measured relative to the tibial plateau. The peak of the intercondylar eminence extends into, but not through, the intercondylar opening in the femoral component, such that resection of the femur to provide clearance for the intercondylar opening and the intercondylar eminence is not required. In the preferred embodiment, an anterior edge of the posterior intercondylar portion of the femoral component contacts the posterior side of the intercondylar eminence after the knee joint has flexed through a substantial portion of its range of flexion to prevent anterior dislocation of the femoral component, relative to the tibial component, as the degree of flexion increases. In an especially preferred embodiment, contact between the edge of the posterior intercondylar portion and the posterior side of the intercondylar eminence occurs after the joint has flexed through approximately 40.degree. of its range of flexion.
As flexion continues beyond the point where the edge of the posterior intercondylar portion contacts the side of the intercondylar eminence, an exterior side, or guide surface, of the posterior intercondylar portion contacts the posterior side of the intercondylar eminence to cause the condylar bearing portions to roll posteriorly on the bearing surfaces of the tibial component. This results in displacement of the point of contact between each bearing portion and its respective bearing surface posteriorly by a predetermined distance. In the preferred embodiment, posterior displacement of this point of contact begins after flexion of the knee joint through approximately 40.degree., and ends after flexion of the joint through approximately 90.degree..
The shape of the resected femur (i.e., the distal cut) conforms to contours which match respective interior surfaces of the condylar bearing portions and the anterior and posterior intercondylar portions of the femoral component. These surfaces are preferably substantially flat, planar surfaces which intersect the substantially coplanar edges of these portions which define the intercondylar opening. Accordingly, the profile of the distal cut is essentially unchanged by the existence of the intercondylar opening in the femoral component, and by the presence of the intercondylar eminence on the tibial component.
In a preferred embodiment of the knee joint of the present invention, the tibial component comprises a tibial base and a tibial insert. The insert is preferably a non-metallic pad having the spaced-apart bearing surfaces and the intercondylar eminence formed on its superior surface. The base comprises a substantially planar platform portion for supporting the tibial insert, and a downwardly extending extension portion for insertion into an opening in the tibia to anchor or secure the platform portion to the tibial plateau. In an especially preferred embodiment, at least a portion of the extension portion is detachably secured to the platform.
The preferred embodiment of the knee joint of the present invention further comprises fastening means for securing the tibial insert to the tibial base, and for securing the detachable extension portion to the tibial platform. In an especially preferred embodiment, a single screw accomplishes both of these functions. The tibial platform may also be provided with one or more laterally spaced openings for receiving additional fasteners (e.g., bone screws) to aid in securing the platform portion to the tibial plateau. When a non-symmetrical detachable extension portion is used, means may be provided to rotationally orient the detachable extension portion, relative to the platform portion.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.